This invention generally relates to beamsteering antenna systems, and more specifically, it relates to a transmitter system in a Traffic/Alert Collision Avoidance System (TCAS) which is particularly suited for directional interrogation of a target aircraft relative to a protected aircraft and methods of operation of the same.
The TCAS equipment located aboard a protected aircraft periodically transmits interrogation signals which are received by transponders located aboard other aircraft, hereinafter referred to as target aircraft, in the vicinity of the protected aircraft. Either Mode S or Mode C altitude reporting transponders are preferred. In reply to the interrogation signals, the target aircraft's transponder transmits a response signal. The TCAS equipment aboard the protected aircraft determines the range of the target aircraft in accordance with the round trip time between transmission of the interrogation signal and receipt of the response signal, taking into account standard delays within the transponder. In addition, if the target aircraft is equipped with a Mode S transponder, the protected aircraft can identify the target aircraft from a unique address assigned each Mode S equipped aircraft.
Information in the response signal, as well as information derived from the response signal, is used by the TCAS equipment to determine whether there is a threat of collision between the protected aircraft and the target aircraft. The response from each target aircraft is processed individually to determine the degree of threat and then, if necessary, an appropriate resolution advisory (climb or descend) is issued by the TCAS equipment to the pilot of the protected aircraft to minimize the degree of threat.
The current generation of TCAS, generally known as TCAS II, must be capable of providing directional interrogations from both top and bottom mounted antennas for surveillance of mode C target aircraft in densities up to 0.3 aircraft per nmi.sup.2. Additionally it has been proposed that the next generation of TCAS, TCAS III, will provide for both horizontal and vertical resolution advisories to provide enhanced positive separation of aircraft. To address increasing densities of aircraft under the TCAS II equipment and the need for enhanced resolution advisory processing speeds and increased horizontal discrimination of intruding aircraft, it is becoming increasingly desirable to provide for enhanced capability of performing directional interrogations of smaller slices of space about the aircraft that may include intruding aircraft. It, therefore, is necessary to maintain proper alignment of the transmitted radiation patterns for TCAS equipment. The directional interrogations allow for focussing the threat calculation algorithms and systems on certain spaces around the protected aircraft. For example, the air space in front of the protected aircraft generally requires more frequent updates than an area behind the aircraft. Directional interrogations essentially limit the location of and number of responses and thereby allowing for quicker processing of transponder responses from the most important air spaces about the aircraft.
TCAS processors transmit interrogations to transponders on the assigned radio frequency channel at 1030 MHz. Because all transponders respond back on the same assigned radio frequency channel at 1090 MHz, it is necessary to employ techniques to segregate transponder responses, in time, such that several responses do not occur at the same time, causing any, or all, responses to be garbled and un-decodable by the TCAS processor.
Ground stations, typically at airports, have been interrogating aircraft transponders for several years. At airport ground stations, segregation of transponder responses is typically accomplished by employing a very large rotating high gain directional antenna with a very narrow beamwidth. Because these large antennas cannot be used on aircraft, other methods had to be employed within the Traffic alert and Collision Avoidance System (TCAS) function employed on aircraft.
On aircraft, typically a small four-element antenna array capable of four directional beams is used. The beam width of this array is relatively wide beam being at least one quadrant. The four directional beams provide some, but not sufficient, segregation in space. For further segregation, a process called whisper/shout is used. Whisper/shout essentially varies TCAS transmitter power level and correspondingly varies TCAS receiver threshold level, to segregate responding transponders in relation to distance from the protected aircraft. A suppression process included within transponders is utilized as a part of this segregation process. Thus, for of each interrogation transmission from a TCAS unit, there is little overlap of responses received back at a TCAS receiver from several transponders in an airspace.
Consequently, there exists a need for improved TCAS transmitter systems for enhancing transmitter phase tuning and further enhancing beamsteering ability for TCAS interrogations.